Textile fabric



Oct. 3, 1939.

C. H. MASLAND, 2D,

TEXTILE 3121c Filed Jan. 9, 1959 s Sheets-Sheet 1 Oct. 3, 1939. c, MASLAND' 2|) 2,174,991

TEXTILE FABRIC Filed Jan. 9, 1939 6 Sheets-Sheet 2 aw W (Pet 3, 1939. c. H; MASLAND, 2D 2,174,991

TEXTILE FABRIC Filed Jan. 9, 1939 6 Shee1; s$heet 3' c. H. MASLAND, 2o

TEXTILE FABRIC Filed Jan. 9, 1933 6 Sheets-Sheet 4 5 5 Idvzfir Oct. 3, 1939. c. H. MASLAND, 20 99 TEXTILE FABRIC Filed Jan. 9, 1939 6 Sheets-Sheet 6 w gfg g g mam.

Patented Oct. 3, 1939 TEXTILE FABRIC Charles Henry Masland, 2nd, near Carlisle, Pa., assignor to C. H. Masland & Sons, 1110., Carlisle, Pa., a corporation of Pennsylvania Application January 9, 1939, Serial No. 250,001

31 Claims.

My invention relates to textile fabrics and to face yarns and artificial fiberstherefor, including both the processes and the products.

This application is a continuation in part of my U. S. patent application, Serial No. 147,883, filed June 12, 1937, for Textile fiber, yarn, fabric, and method.

A purpose of the invention is to reduce the dust-retention of textiles and particularly of face yarns (such as pile yarns) which are normally cleaned by brushing, sweeping, beating or the vacuum cleaner. The widest application of the invention is believed to be in housefurnishing fabrics such as upholstery fabrics and floor cov erings, whether used in buildings, vehicles or the like.

A further purpose is to form fabrics subject to washing at infrequent intervals only, whose fiber cross-sectional outlines and fiber sizes are such that there will be low dust retention.

A further purpose is substantially to avoid curves on a radius less than 4 microns in the cross-sectional outline of an artificial textile fiber, desirably substantially avoiding curves on a radius of less than 6 microns.

A further purpose is substantially to reduce the dust-retention by a fabric of'the type subject to washing at infrequent intervals only, by

1 making the face yam-of the fabric of at least- 20% (preferably at least 50%, and in many instances substantially 100%) of artificial textile fiber of average diameter between 27 and '75 microns (for floor coverings, preferably between 37 and '15 microns, superior results being obtained as for average fiber diameters between 3'7 and 60 'microns, and most desirably between 3'7 and 52 microns), less than 20% (preferably less than 10% and most desirably substantially of whose cross-sectional periphery is on a radius of curvature between 4 microns and 0.4 micron (preferably less than this percentage between a radius of 6 microns and 0.4 microns, whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which include any arc on a radius less than 4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain dust. ,I

A further purpose is to make the face yarn of a fabric subject to washing at infrequent inter-. vals only, partially or entirely from an artificial textile fiber having a cross section which is a p smooth closed curve of average fiber diameter between 2'7 and '75 microns.

A further purpose is to make the face yarn of a fabric subject to washing at infrequent intervals only, from a cellulose rayon whose fiber size and cross-sectional periphery have been designed in accordance with the principles of the invention. Regenerated cellulose rayons such as viscose and cuprammonium may be used in the invention. The application best illustrative of 10 the principlesof the invention appears to be in viscose, where the usual commercial fibers deviate so radically from the requirements of the invention. Since the coagulation of cuprammonium rayon is more even toward the center, the departure of cuprammonium rayon of the present invention from prior art cuprammonium rayon in the sizes under discussion is less marked. The invention may also be applied to rayons consisting of organic derivatives of cellulose, such as cellulose esters (cellulose acetate, cellulose formate, cellulose butyrate, cellulose propionate, nitrocellulose, etc.) and cellulose ethers (methyl cellulose, ethyl cellulose, benzyl cellulose, etc.) I find that the principles of the invention may also .25 to advantage be applied to naturally-occurringprotein rayons, such as casein rayon and soya bean rayon.

A further purpose is to reduce dust retention I by an. artificial textile fiber of undulating crosssection outline and average diameter between 2'7 and '75 microns, by forming less than 20% (preferably less than L0% and most desirably sub stantially 0%) of the cross-sectional periphery on a radius of curvature between 4 microns and 0.4 (and preferably less than the specified percentage on .a radius of curvature between 6 microns and 0.4 micron), and avoiding objectionable reentrant curves as already explained.

A further purpose is to produce fabrics of 40 the type subjected to washing at infrequent intervals only, from continuous filaments or artificial staple fibers of low dust retention by forming the fiber sizes and cross-sectional peripheries of the fibers in accordance with the invention.

A further purpose is to produce face yarns such as pile yarns for pile fabrics, for example carpet yarns, and also pile fabrics, such as rugs, carpets, draperies, and upholstery fabrics, which are of markedly low dust retention.

A further purpose is to produce suitings of re: 1 duced dust retention in accordance with the principles of the invention.

Further purposes appeanln the specification and in the claims.

The invention relates both to the processes and to the products.

Figures 1 to 7 show greatly enlarged diagrammatic sections of artificial textile fibers.

Figure 8 is a perspective view showing a conventional yarn and Figures 9, 10 and 11 are sectional views showing conventional pile fabrics,

particle size is from 0.8 to 1.5 microns (a micron is of course a millionth of a meter).

As used herein, textile fibers, threads, yarns and fabrics include those employed in or produced by weaving, knitting or other textile processes. Typical examples of textile fibers are wool, flax, cotton, alpaca, mohair, camels hair, silk and rayon. An artificial textile fiber is an individual filament having the resilience and other properties suiting it to be handled as a textile fiber and produced artificially. Materials such as metallic wires are not to be regarded as artificial textile fibers. Artificial textile fibers are generally of organic as distinguished from inorganic composition, and in most cases consist of regenerated cellulose or an organic derivative of cellulose. Rayons such as cellulose rayons and protein rayons are typical examples of artificial textile fibers.

The subject matter of this application is related to that of the copending Porter & Barker U.'S. patent'application, Serial No. 180,026, filed Dec. 15,1937, for Yarn and fabric, Patent No. 2,143,574, granted Jan. 10, 1939, and the disclosure of this Porter and Barker application is incorporated herein by reference.

In the past, artificial textile fibers of about 22 microns average diameter (approximately denier) and irregular cross section, have been used with wool or mohair, in blends, for fabrics subject to washing at infrequent intervals only, such as carpets, rugs and upholstery fabrics. The proportion of such artificial fibers used in the blend has been limited, due in part to the excessive dust retention of the irregular cross section viscose fiber used. Certain washable fabrics such as small bathroom mats, have had piles consisting entirely of such 22 micron (approximately 5 denier) rayon fiber of irregular cross section. Quite smooth 1 round cuprammonium fibers of small sizes, exhibiting high. dust retention, have been produced and used in various washable fabrics. Also, quite smooth round artificial horsehair, not spun into yarn, has been made.

The present inventor has conducted an extensive study of the problem of dust retention by textile fabrics.

The actual experimental results obtained by .the present inventor with artificial textile fibers produced for experimental purposes have seemed at first to make the cause of dust retention more obscure. It is well known that existing commercial artificial textile fibers such as rayons, are objectionable because of dust retention, so that they may not be used in fabrics subjected to washing at infrequent intervals only, except as minor constituents. it has long been recognized that prior art fibers of cellulose and organic derivatives of cellulose, soil more readily and retain dust more tenaciously than wool. The behavior of the experimental artificial textile fibers tested by the present inventor seemed anomalous, some showing high dust retention and some showing a freedom from dust retention equal to wool.

A number of possible explanations of the phenomenon have been studied. It has been sup posed that the retention of dust may be due primarily to an adhesive film on the fiber. However, experiments show that very carefully cleaned prior art artificial cellulose and wool fibers exhibit similar differences in dust retention to those of the commercially clean fibers.

A moisture film on the artificial fiber has been suggested as the cause, but when those artificial textile fibers which show high dust retention are carefully dried, the dust retention is found to increase instead of being reduced, as would be the case if a moisture film was a primary cause of dust retention. It has been thought that possibly opposite electrostatic charges between the dust particles and the fibers were the major causes in dust retention. However, the electrostatic charges on both the dust particles and the fibers change both in sign and amount with weather conditions and previous treatment, and, in the case of the dust, with the chemical character. Yet these electrostatic changes do not produce any major change in dust retention. Therefore the electrostatic charge on the fiber as a whole cannot be a major consideration.

The chemical composition of the artificial fiber has been considered as a possible cause. The surface of wool fibers is diflicultly wet by water, a polar liquid, that is, the angle of contact is large. But wool is readily wet by carbon tetrachloride, a nonpolar liquid, that is, the angle of contact approaches zero. Artificial cellulose fibers are polar and have a lower angle of contact against (are more readily wet by) water than against carbon tetrachloride. Dust from many sources is nonpolar; it has a large angle of contact against water and a very small angle of contact against carbon tetrachloride. Yet experiments show that the nonpolar dust adheres better to certain commercial artificial cellulose fibers, polar, than to nonpolar wool. Artificial cellulose fibers in some cases exhibit high dust retention, and in other cases do not. Therefore my present knowledge indicates that chemical composition as we ordinarily use the term is not a major factor in dust retention.

Again it may be supposed that fiber size (average outside diameter) is the consideration which controls dust retention. This view would seem to be supported by the fact that in cases in which a coarse natural fiber does not objectionably retain dust, the same material where very fine exhibits high dust retention. Also very fine commercial artificial cellulose fibers invariably exhibit high dust retention. But, unfortunately for this explanation, some extremely coarse experimental artificial fibers also show high dust retention, while other experimental fibers of the same size manufactured for the present inventor do not.

The retention of dust by fabrics is the result of equilibrium between forces tending to hold particles of dust in contact with the fibers and forces tending to remove particles from the fibers. The present invention is concerned with fabrics subJ'ect to washing at infrequent intervals only. and therefore regularly cleaned by household methods, such as brushing, sweeping, beating and vacuum cleaning. This class includes the housefurnishing fabrics, such as floor coverings like rugs and carpets, and such as upholstery fabrics like upholstery cloth, plush; velvet, tapestry, draperies and hangings, whether for buildings or vehicles. Most housefurnishing fabrics are pile fabrics, although some are flat fabrics. There are also clothing fabrics which are subject to washing at infrequent intervals only, such as suitings, corduroys and velvets, to which this invention applies. Fabrics of the above types ar very seldom washed.

Assuming any household cleaning means of constant value such as beating, brushing, sweeping or the vacuum cleaner, for removing particles of dirt from fabrics subjected to washing at infrequent intervals only, dust retention by the-face yarn of the fabric, that is by the pile yarn .of apile fabric or by any yarn carried to the face of the fabric in a fiat fabric, may decrease due to:

1. A reduction in the magnitudes of forces tending to hold' particles against the fibers. These forces may include the electrical moment of the molecules composing the surface, static electricity and adhesive films. The reduction in the magnitudes of these forces is not within the scope of thepresent invention.

2. Making the face yarn wholly or in part of fibers of such size and cross-sectional contour that the forces tending to hold the particles against the fibers as in (1) have the least possible effect.

3. Making fibers as well as yarns and fabrics "of such character that the dirt removing forces can reach the dirt. This requires that the fibers be disturbed and moved incleaning and that the fabric be constructed so that the fibers do not mat. This also requires that objectionable reentrant curves in the cross-sectional contour of the fiber be avoided, as they form channels in which dirt particles can avoid contact with the dirt-removing means, leaving lines of soil on the fiber.

As a result .of extensive microscopic studies, the present inventor has discovered that crosssectional outline is a critical factor in dust retention by artificial textile fibers, as it apparently gives effect to surface forces, at the surfaces of the dust and the fibers. The ranges of action of the surface forces are very small. The space relation of the molecules at the surface to neighboring molecules appears to be a critical factor in dust retention. 1

In order that the forces which tend to hold particles against the fibers shall not cause excessive dust retention, the cross-sectional contour of the artificial textile fiber should have the following characteristics:

1. Less than 20% of the cross-sectional periphery of the fiber (preferably less than 10% and most desirably substantially 0%) should be on a radius of curvature between 4 microns and 0.4 micron (preferably less than this percentage between a radius of 6 microns and 0.4 micron.) If the cross-sectional contour of the fiber contains excessive curves on a radius within this critical range, the forces at the fiber surface cause the fiber to retain excessive dust, while if the radius of curvature is greater than this critical range, the fiber will not retain dust objectionably, other factors as noted herein being under control. Extensive experiments indicate that the critical upper limit of the objectionable radius of curvature is of the order of 4 microns, probably slightly less than 4 microns. At any rate, where the entire cross-sectional contour of the fiber is on a radius greater than 4 microns, one critical factor which will assure low' dust retention is present, and where the radius of curvature is greater than 6 microns, the assurance of low dust retention from the standpoint if this factor is increased.- A radius of curvature less than 0.4 micron is less than the most probable particle size of the dust, and so cannot retain any ap preciable density of soil, and any soil which it might retain would not in all likelihood be optically dense.

2. If the fiber is of small diameter, it will pick up dust regardless of whether its surface be smooth or irregular. This explains why fine natural and artificial textile fibers retain dust objectionally in floor coverings, upholstery fabrics and suitings. The size of the fiber should be above a certain minimum below which all fibers having the same surface characteristics are subject to objectionable dust retention. As noted below, the minimum fiber size which will produce a dust retention as favorable as that of carpet wool is 27 microns average diameter (about 8 denier in regenerated cellulose).

All artificial textile fibers used in the present invention should preferably be uniform in crosssectional area, although the periphery may vary at different points along the fibers. A convenient way to measure average diameter is to circumscrlbe a circle around the fiber, and then lay out eight .equally circumferentially spaced circle diameters, measuring the width of the fiber on each diameter and striking an average.

3. The cross-sectional periphery of the fiber should be substantially free from reentrant curves which include any arc on a radius less than 4 microns and which have a depth greater than 0.5 micron, and substantially free from reentrant curves of depth greater than 0.5 micron and whose mouth opening is less than five times the depth. By this means, we avoid the presence of reentrants deep enough to retain appreciable soil and possessing either of the characteristics likely to cause soil retention, namely (a) curves of small radius which will readily retain dust, and (b) relative depth of the reentrants compared to the mouth opening which will render application of the dust-removing forces very difficult.

Figures 13 to 16a illustrate the features of the invention. The figures numbered 13 show cuprammonium rayon, the figures numbered 14 and 15 show viscose rayon, and the figures numbered 16 show wool. Experimental carpet samwool pile covering half of the area and a pile consisting of a blend of rayon with wool in the other half of the area.

The wool piles were all alike and the piles containing rayon differedonly in the character of the rayon and the percentages of rayon in the blends. These samples were subjected to uniform soilage conditions in the living room of a home for five weeks, during which time all samples were cleaned alike at usual intervals of household cleaning methods. Microscopic studies were made of representative rayon fibers contained in the rayon portion of the carpet samples before and after soilage, and for comparison the soilage of the wool fibers in the wool portions of the carpet samples was also studied. All of these views were taken at a magnification of 600 diameters. v Figures 13 and 130. show cuprammonium fibers taken from a pile containing,70% cuprammonium 5 rayon and 30% wool. Figure 13 is a longitudinal view of a typical cuprammonium fiber before soilage and Figure 13a is a longitudinal view of a typical cuprammonium fiber after soilage. The fiber cross section of the cuprammonium is somewhat as shown in Figure 1, there being a large number of irregularities and frequent reentrant angles. In Figure 13a the soilage is very evident, particularly in the longitudinal channels formed by the' reentrant angles. 15 The fibers shown in Figures 14 and 14a were taken froma pile yarn consisting of a blend of 70% of irregular cross section viscose and 30% of wool. The fiber cross section of the viscose was much as shown in Figure 2, having numergo ous 'reentrant angles. Figure 14 shows a longitudinal view of a typical irregular rayon fiber taken from the unsoiled pile. Figure 14a shows a longitudinal view of a typical irregular rayon fiber after completion of the soilage tests. The soilage is very noticeable, particularly in the longitudinal channels formed by the reentrant angles.

For Figures 15 and 150, the pile yarn was composed of 70% of a special smooth contour round so cross section viscose rayon and of wool. The cross sections of the viscose fibers were much as shown in Figure 4. Figure 15 is a. longitudinal view of a typical smooth contour rayon fiber taken from the pile before soilage, and Figure 15a shows a longitudinal view of such a fiber after completion of the soilage tests. Soilage may be observed, but it is much less pronounced than in the case of Figures 13a or 140, and is not oriented in any longitudinal configuration,

except in small-sections where due to manufacturing imperfections the contour is not perfectly smooth.

Figure 16 shows in longitudinal view a typical unsoiled wool fiber. Figure 16a is a longitudi- 5 nal view of such a wool fiber after soilage.

The following experiments indicate quantitatively the soiling characteristics of various fibers in fabrics. The dust retention was measuredby theparticle count method. Where the fiber is o present in the face yarn of a fabric, the fabric should be thoroughly vacuum cleaned before samplin'g, and then representative portions of the fabric picked apart, rejecting the backing fibers, and selecting a 5 gram sample face of fibers. 55 A standard scouring solution consisting of 0.5% Gardinol W. Ar Paste (detergent) in distilled water is used. To 250 cc. of securing solution'ata temperature of 130 F., contained in a warmed Mason jar, the sample of fiber is added 00 and the cap-tightly screwed on. The far and contents are shaken vigorously for 5' minutes. At the end of this time'the jar is opened and the scouring liquor poured off into a clean .600 cc. beaker through a coarsescreen (16 wires per erinch) on which should beoolleoted most of the fibersias well as alargepartof the foam. A small swatch of the fibers from the screen should then be dried by pressing between paper towels and theflbers mounted in glycerine for examina- 70 tlon under the microscope for soil'particles which have. not beenremoved by the scouring" liquor.

I 15 In counting the soil particles an improved Neubauer-ruled Levy counting chamber and cover glass should be carefully washed, rinsed and dried and then wiped with lens paper. The cover glass should then be put in place on the chamber. Be-

fore sampling the scouring liquor it shouldbe -5 cooled to room temperature, then thoroughly mixed by pouring from one beaker to another ten times, and, a small medicine dropper pipette I should be filled from the center of the beaker half way to the bottom of the solution while the 10 liquor is still turbulent. After quickly and carefully wiping the outside of the pipette dry, then flushing the tip, a drop or less of the scouring liquor should be rapidly transferred to the ground glass ramp of the cylinder which is in position 15 on the stage of the microscope, and then drawn into the chamber by capillarity. The filled cham'- ber should be allowed to rest for fifteen minutes in order that the particles will gravitate to the ruled surfaces of the chamber after which the go count should be made. The particles in a number of squares of the 25 composing the center square millimeter should be counted and the average number of particles per square (area-V square millimeter) computed. The counts should as be made using 10X objective, 15X eyepiece, 160 millimeters tube length, with the upper element of the Abbe condenser removed and the illuminating system just slightly out of critical focus. With the above set-up particles as small as 0.2 80 micron may be counted fairly easily.

In order to make the calculations, the average number of particles per square, ,4 sq. mm. in area, as found above, should be multiplied by 12.5 in order to obtain a figure which is the nums5 ber of millions of particles per gram of face yarn. To arrive at this figure it is necessary to consider only the conditions of the experiment. The particles are counted per square of ,5 sq. mm. area, the cell is 0 .l millimeter deep; hencethe volume. 0 of liquor in which the particles are counted is 0.004 cubic millimeter. Because the volume of the scour liquor is 250 cubic centimeters and the weight of the fiber sample is 5 grams, it is necessary to find the number of particles in 50 cubic centimeters inorder to find the number of particles per gram of face yarn. It will be seen that 0.004 cubic millimeter is equal to 0.000004 cubic centimeter and cubic centimeters is ori2.5x 10 times as great as the volume counted; hence to obtain the number of particles in millions per, gram of face yarn, multiply the number of particles counted in a square by 12.5.

A typical example of the application of the method is-as follows: A test carpet was made up which is composed of strips of 1) carpet wool,

(2) a blend of.30% carpet-wool with smooth sweeping and vacuum cleaning. as in ordinary 70 household practice. Before sampling, the soiled carpet was thoroughly vacuum' cleaned, afterwhich samples of approximately 4' x 4 inches were cut out of the different sections, approximately afoot from the end of the carpet. These samples 7 were then taken apart, separating and saving the face yarns. The results were as follows:

Blend of wool Blend of W001 W001 and smooth and irregular circular cross cross section section rayon rayon Number of squares in which ticles were oounte 5 5 3 Average number of par- 184. 4 187. 2 3 ticles per square 102. 8 188. 8 0 Average number of particles in terms of mil- 2050 2340 7070 lions of particles per 2040 2300 7280 gram of face yarn 1. 01 1. l5 3. 48 Relative soil L 00 1 14 3' 58 Average 1.005 1. 145 3. 53 Relative soil, with wool on a basis of 1 1.00 1.14 3. 51

From the above experimental data, it will be seen that in this case the face yarn containing only 50% of irregular cross section rayon soiled more than three times as badly as 100% carpet wool.

In some cases, the testsof the smooth round cross section rayon in the fabric show markedly lower dust retention than wool.

In order to eliminate the effects of matting of the fibers in fabrics and yarns on the relative dirt retention by fibers of different cross-sectional contours, unspun fibers were soiled by mixing one gram of fiber with one-half gram of dust in a ball millfor one hour. The fibers were then placed in a container having a fine mesh screen top and bottom and were agitated thoroughly for ten minutes by a stream of air issuing from a nozzle at constant velocity. The fiber sample taken for test was one-half gram.

The dust used was the same in all tests and had the following characteristics:

Range of particle size, 0.3 to 3.5 microns Most probable particle size, 0.8 to 1.5 microns Water soluble, 8.98%

Ash, 63.10%

The composition of the ash was chiefiy calcium and other alkaline earth metals, together with some iron and aluminum, in the formof carbonates, silicates and oxides. Carlisle, Pennsylvania, but does not differ materially from dust found in the vacuum cleaners at widely separated points such as Philadelphia, Oklahoma, New York and San Francisco which has been analyzed.

Figures 1'7 and 18 show dust retention characteristics of smooth round fibers of regenerated 'cellulose, substantially free from curves on a tated into the plane of the paper, so that it extends down from the axis of abscissae (X-axis) Figure 1'7 plots dust retention by the particle count method as the ordinate (Y-axis), size of textile fiber undergoing tests as the abscissae (X-axis) and surface per gram of fiber in square centimeters on the Z-axis. By study of curve .A, for the smooth round regenerated cellulose fibers, it willbe evident that fibers of this cross section exhibit very high soil retention in low deniers and very low soil retention in high deniers. Qurve B represents the dust retention of one gram This dust came from of typical carpet wool fibers. The point on the axis of ordinates (Y-axis) corresponding to the soil retention of carpet wool has been extended horizontally until it intercepts curve A for round smooth regenerated cellulose fibers at C, approximately 8 denier (27 microns average fiber diameter).

This indicates that even smooth contour regenerated cellulose fibers of the electrical moment characteristics of the fibers now produced, and finer than 27 microns average diameter (about 8 denier) having such a great external area that their dust retention in the loose fiber as compared to carpet wool loose fiber is too great to make them of practical value where dust retention is a serious factor. This is true even though the ratio of dust retention to area is nearly constant down to about 1.25 denier. Thus an average fiber diameter of 27 microns (about 8 denier) may be set as the practical lower limit to which the principles of the invention may be applied in regenerated cellulose fiber as commercially produced.

It will be evident that above the point C, the curve A is generally extending vertically and below the point C, the curve A is generally extending horizontally, the point C being an approximate indication of the break in curve A.

The principles of the invention may be usefully applied in fibers as large as 75 microns average fiber diameter (about 60 denierLbeyond which size fiber can hardly be spun into face yarn. In so far as floor coverings are concerned, the preferable limit for average fiber diameter for application of the invention is between 37 and '15 microns, superior results being obtained for average fiber diameters between 37 and 60 microns, and even better results for average fiber diameters between 37 and 52 microns.

' An excellent fiber diameter for floor coverings is about 46 microns. For suitings and certain upholstery fabrics, the preferable fiber size range is between 27 and 37 microns.

It will be understood that in coarse fibers, of average diameter above 37 microns, considerable irregularity is possible without having a radius of curvature less than 4 microns.

Figure 18 plots dirt retention by the particle count method as ordinates (Y-axis) and surface per gram of' fiber in square centimeters as abscissae (X-axis). In the case of irregular cross-sectional contours, the external area was determined by measuring the perimeters of many fibers on greatly enlarged photomicrographs with a map measure, and multiplying by the length. The regenerated cellulose fibers of curve A are the same as those considered in curve D.

Curve D is substantially a straight line. study of curve D it will be observed that for the lower deniers of round smooth cross section regenerated cellulose fibers, the external. surface is high and corresponds to high soil retention, as in curve A above the point C. ,On the other hand, the higher deniers of round smooth cross section regenerated cellulose fibers exhibit low soil retention as in the case of the portion of curve A below the point C.

' ably above the curve D for smooth round cross section textile fibers and serves to illustrate the dust retaining characteristics of reentrant curves which have a depth greater than 0.5 micron and which include any are on a'radius less than 4 microns, and reentrant curves of depth greater than 0.5 microns whose mouth opening is less than five times the depth. The average diameter of this fiber is about 34 microns.

The point F on Figure 18 shows the dust retention characteristics of 10 denier irregular cross section viscose fibers plotted against the surface per gram of fiber in square centimeters. The cross-sectional contour is somewhat as shown in Figure 2. This point is considerably above the curve D for smooth contour fibers'and is also above the point E for objectionable reentrant curves in the cuprammonium fiber, thus showing the tremendous increase in dirt retention due to a cross-sectional contour having excessive curves on a radius between 4 microns and 0.4 micron," over a smooth fiber cross-sectional contour of the same exterior area, freefrom'curves on a radius less than 4 microns.

In Figures 1'7 and 18, the Z-axis curve is in each case surface per gram of fiber in square centimeters plotted against fiber size in denier (or vice versa). Curves G and G showthe surface-denier relationships for the dust retentions shown by curves A and D. The point C applied to curves D, G and G indicates the point beyond which dust retention is excessive. Thus on curves G and G, dust retention is excessive between the points C and H, but is not objectionable for fibers on the curve between the points C and J and beyond J up to '75 microns average fiber diameter. The curves G'and G follow the approximate'relation Surface per gram in square centimeters Surface per gram in square /7,440,000

centimeters \I Denier By study of Figure 1'7 it will be observed that fiber having the surface characteristics of those I under discussion cannot be employed com-.

mercially in sizes lessthan 27 microns average diameter where dust retention is a serious factor. Any fiber showing a dust retention of less than 15,000x10 particles per gram of fiber by the particle count method can satisfactorily be. used- The presence of occasional curves on a radius of.

curvature between 4 microns and 0.4 micron (or between 6 microns and 0.4 micron) will not cause the fiber to .retain dust objectionably providing such curves do not amount to more than 20% of the cross sectional periphery, and do not result in objectionable reentrants. The curves of small radius; i. e., less than 0.4 micron, are difiicult to avoid in manufacturing practice and do not result in optically dense soil, although they will desirably be absent. Less than 10% and me!- erably' substantially 0% of such curves of radius between 4 microns (preferably 6 microns) and 0.4 micron will preferably be present.

For best results the cross-sectional outline should be substantially free from curves of radius less than 4 microns and preferably substantially free from curves of radius less than 6 microns.

If the cross-sectional outline is a smooth closed curve such as a circle or an ellipse .of average diameter between 2'7- and 75 microns, or one of the narrower preferable ranges within this wider range, substantially free from curves on a radius 'below 4 microns (preferably 6 microns), the most desirable. condition will exist.

Figures 1 to 7 show the cross-sectional outline of artificial textile fibers. These fibers will desirably be made of cellulose rayons. Regenerated cellulose rayon such as viscose and cuprammonium may be used, perhaps the greatest field for the invention being in viscose because the viscose fiber at present is so radically divergent from the principles of the invention. As cuprammonium rayon coagulates more evenly toward the center than viscose rayon, the departure from the prior art fibers of the size in question will be less marked in cuprammonium than in viscose. Besides regenerated cellulose rayon. other cellulose rayons such as organic derivatives of cellulose including cellulose ester rayons (cellulose acetate, cellulose formate, celluose butyrate, cellulose propionate, nitrocellulose, etc.) and cellulose ether rayons (methyl cellulose, ethyl cellulose, benzyl cellulose, etc.) may be used. I also find that satisfactory results may be obtained with naturally-occurring-protein rayons, such as casein and soya bean rayons. Those rayon materials which are regenerated cellulose or organic derivatives of cellulose are all generically described as artificial cellulose fibers.

In Figure 1 the cross-sectional outline 20 is very irregular. Besides numerous objectionable reentrant angles, there are many curves 2| which, in the actual fiber, are on a radius between 4 microns and 0.4 micron. The cross section of Figure 1 is accordingly to be avoided. It corresponds with commercial cuprammonium rayon of 12 denier which has retained dust very objectionably in the prior art.

Figure 2 shows a very irregular cross-sectional outline 22. which is rather characteristic of certain viscose fibers which have exhibited high dust retention. The curves on the surface are shown at 23. This cross section is of course to be avoided. I

Figure 3 shows a fiber whose cross-sectional outline 2| consists mainly of curves 2! of radius larger than 4 microns. This fiber will be acceptable in average diameters between 27 and '75 microns as it is. free from objectionable reentrants, and less than 20% of the cross-sectional periphery is on a radius between 4 microns and 0.4 micron.

Figure 4 shows a fiber of smooth generally circular cross-sectional outline 26 which is practical-' iy all on the same radius of curvature. This fiber is preferable to the various other forms as it exhibits practically minimal dust retention in'any size between 27 and 75 microns average diameter for any given fiber surface composition. In Figure 5 the cross-sectional outline 21 consists of curves 28 which are on a relatively large radius at the sides and a relatively small dimension for fear of cork-screwing when twisted and crushing or curling under pressure in the pile, thus increasing dust retention through matting of the fibers. In general, as explained in the Porter and Barker application above referred to, the minimum diameter should not be less than 40% of the maximum diameter of the fiber at a given point.

While it is most desirable that the fibers be .smooth and round, the fibers may be of undulating cross section as shown in Figure 3 providing less than 20% of themross-sectional periphery is on a radius between 4 microns and 0.4 micron (preferably less than 10% and most desirably about and providing objectionable reentrant angles are absent.

Reentrant angles or curves are in general objectionable and therefore to be avoided in a fiber cross section in accordance with the invention. There are two kinds of reentrant curves which are not objectionable from the standpoint of dust retention. The first unobjectionable type of reentrant is one which is so shallow that it cannot retain optically dense soil and therefore cannot have any appreciable optical effect. The second type of unobjectionable reentrant is one which is so wide and open and so free from curves of small radius as to be readily accessible to the meaning means, and without marked tendency to retain dust.

It has been determined experimentally that reentrant angles or pockets of depth less than 0.5 micron are not capable of retaining optically dense soil and therefore of causing difiiculty. Hence up to 20% of the cross-sectional contour may be in the formof curves of radius between 4 microns (or 6 microns) and 0.4 micron present in reentrants of depth less than 0.5 micron or present in convex curves not giving rise to reentrants. In measuring the depth of a reentrant, a line should be drawn across the mouth from the nearest point of tangency on one side to the nearest point of tangencyon the other side and the distance measured from the bottom of the reentrant to this line. For example, in Figure 6 the depth of the reentrant 29 is the distance 29 from the bottom of the reentrant to the line 30 connecting the point of tangency 3| on one side with the nearest point of tangency 3! on the other side.

Reentrant curves are particularly objectionable figure, or in the form of meeting fissure walls 32, Figure 7, which at the point of meeting produce a curve of very small radius progressively and slowly widening into a curve of larger radius, and causing great dust retention.

, 73 Another standpoint from which reentrants may be objectionable even if all parts of the reentrant curve are on a radius greater than 4 microns, the formation of relatively narrow deep channels whose depth exceeds 0.5 micron. Experiment indicates that such reentrants are dust-retaining when narrow and deep, but not 'when shallow and wide, the critical factor being the accessibility to the dust-removing means. Investigation indicates thatin the case of a reentrant whose depth exceeds 0.5 micron, in orde'r that it may be free from objection, the width or opening at the mouth should be more than five times the depth. The width at the mouth is determined by drawing a line between the nearest points of tangency to the curves on each side of the mouth and measuring the distance between these nearest points of tangency. For example, in the reentrant shown in Figure 3, the mouth opening 30' is more than five times the depth 33' and the reentrant is not substantially harmful, whereas the reentrant 29 of Figure 6 whose mouth opening 30 is much less than five times the depth 33 is very objectionable. The same is true of the reentrants in Figure 7. On the question of the relation of depth to mouth opening, reentrants not as deep as 0.5 micron may be disregarded.

Artificial fiber ranging between 3'7 and 75 microns average diameter, free from objectionable reentrants 'as above set forth and having less than 20% of the cross-sectional periphery on a radius between 4 microns and 0.4 micron is very desirable for pile fabrics such as floor coverings, including rugs and carpets, and such as pile upholstery fabrics including plush, velvet, tapestry, upholstery cloth, hangings and draperies. It will be evident from Figure 17 that the inherent dust retention of a fiber in the range from 37 to "75 microns, and which conforms to the principles of the invention, will be only about two-thirds that of a smooth round fiber of average diameter as low as 27 microns. Because of this lower dust retention under ordinary household cleaning methods, fibers which conform to the invention and range in size between 3'? and '75 microns are particularly desirable for carpets and rugs, which are subjected to very hard usage. It has been found that such carpets and rugs may be cleaned by brushing, sweeping, beating or the vacuum cleaner just as readily orin fact more readily than carpets and rugs of carpet wool, without being so costly or diificult to obtain.

The term yarn as used herein is broad enough to include threads.

The invention is expected to find its greatest use in staple fiber, although it may be applied to fabrics made of continuous filaments or mixtures of continuous filaments and staple fiber. The staple fiber may for example be from to 12 or 14 inches in length, preferably about 4 or inches. Satisfactory fabrics may be made using 100% of artificial staple fiber in the face yarn, although in many cases blends with from 80% to 0% of natural fiber, such as wool, alpaca, mohair or camels hair will be used. For floor coverings, such natural fibers will be coarse,

- such as carpet wool, alpaca, mohair or camels micron, 25% 37 micron, and 25% 48 micron artificial fiber as described in the present application, with 25% of wool. Less than 20% of round smooth artificial staple fiber does not impart any distinct improvement to the fabric. If desired, fine fibers (5 denier or less) may be added to extents of say 15% to increase spinnability.

The invention is useful in both woven and knitted fabrics of the class subject to washing at infrequent intervals only, and in both fiat and pile fabrics. Figure 8 shows yarn 34 conventional in construction, consisting of plies 35, 35' and 35 suitably of twisted artificial staple fiber, whose cross-sectional contours and dimensions conform to the principles of the invention. Figures 9, 10 and 11 show pile fabrics 36, 3'! and 38 whose pile yarns 39, 40 and 4| conform to the invention, while Figure 12 shows a flat, fabric 42 whose face yarn 43 conforms to the invention. The weaves in all cases are conventional, Figure 9 being a carpet, Figure 10 being a plush, Figure 11 being an upholstery cloth and Figure 12 being a suiting.

It will be understood that the artificial fibers used will desirably be substantially uniform in fiber cross-sectonal area, within the limits possible in rayon manufacturing technique. Also desirably the artificial fibers used in the present invention will be solid rather than hollow.

In making the artificial fiber of. the present invention it is possible to use ordinary technique of rayon manufacture, simply slowing the coagulation to avoid or reduce the shrinkage efiects which result in irregular cross sections. This procedure for obtaining smooth round cross sections is well understood by rayon manufacturers. See Reinthaler, Artificial Silk (Chapman 8: Hall, 1928) pages 138 to 147; Hottenroth, Artificial Silk (Pitman & Sons, 1928) pages 354 to 381; Wheeler, The Manufacture of Artificial Silk (1928) pages 84 to 87; British Celanese British Patent 318,643; Dreyfus British Patent 217,287;

Dreyfus U. S. Patent 2,045,346; Dreyfus, U. S. Patent 2,059,322; Dreyfus U. S. Patent 1,865,358; Dreyfus U. S. Patent 1,688,531 and Stone U. S. Patent 2,000,047.

It will be evident that the widest use of the .invention will probably be in artificial cellulose sirably between 37 and 60 microns and most desirably between 37 and 52 microns), should have less than 20% of its cross-sectional contour (preferably less than 10% and most desirably 0%) on a radius between 4 microns and 0.4 micron (desirably less than this percentage between 6 microns and 0.4 micron), should be substantially free from reentrant curves which have a depth greater than 0.5 micron and which include any are on a radiusless than 4 microns,

and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth.

' or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the structure shown, and I, therefore, claim all such in so far as they fall within the reasonable spirit and scope of my invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A fabric which is of the type subject to washing at infrequent intervals only and whose face yarn is made of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 27 and microns, less than 20% 'of whose cross-sectional periphery is ona radius of curvature between 4 microns and 0.4 micron, whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves includeany arc on a radius less than 4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain dust.

I 2. A fabric which is the type subject to washing at infrequent intervals only and whose face yarn is made of at least 20% of cellulose artificial textile fiber of average diameter between 27 and 75 microns, less than 20% of whose crosssectional periphery isv on a radius of curvature between 4 microns and 0.4 micron, whose crosssectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on a radius less than 4 microns, and substantially free from reentrant curves of depth greater than' 0.5 micron whose mouth opening isless than five times the depth, the face yarn possessing a' reduced tendency to retain dust.

3. A fabric which is of the type subject to washing at infrequent intervals only and whose face yarn is made of at least 20% of regenerated cellulose artificial textile fiber of average diameter between 27 and 75 microns, less than 20% of whose cross-sectional periphery is on a radius of curvature between 4 microns and 0.4 micron,

whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on ,a radius less than 4 microns, and substantially free from reentrant-curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain dus 4. A fabric which is of the type subject to washing at infrequent intervals only and whose face yarn is made of at least 20% of viscose artificial textile fiber of average diameter between 27 and 75 microns, less than 20% of whose crosssectional periphery is on a radius of curvature between 4 microns and 0.4 micron, whose crosssectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on a radius less than 4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less thanfive times the depth, the face yarn possessing a reduced tendency to retain dust.

5. A fabric which is of the type subject to $5,174,991 washing at infrequent intervals only and whose face yarn is made of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 27 and 75 microns and which fiber substantially conforms to the condition:

Surface per gram in 11,310,000 square cn l DenierXspecific gravity 6. A fabric which is of the type subject to washing at infrequent intervals only and whose face yarn consists of artificial textile fiber of the class consisting of cellulose fibers and-naturally-occurrlng-protein fibers and of average diameter between 27 and 75 microns and which fiber substantially conforms to the condition:

Surface per gram in square /7,440,000

centimeters Denier 8. A fabric which is of the type subject to washing at infrequent intervals only and whose face yarn is made of at least 20% of viscose artificial textile fiber of average diameter between 27 and 75 microns and which fiber substantially conforms to the condition:

Surface per gram in square 7,440,000 centimeters D i 9. A fabric which is of the type subject to washing at infrequent intervals only and whose face yarn is made of atleast 20% of artificial textile. fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 27 and 75 microns and which fiber in loose form has a.'dust retention of less than 15,000 particles per gram by the particle count method.

, 10. A fabric which is of the type subject to washing at infrequent intervals only and whose face yarn is made of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 37 and 75 microns, less than 20% of whose cross-sectional periphery isv on a radius of curvature between 4 microns and 0.4 micron, whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on a radius less than-4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain dust.

11. A fabric which is of the type subject to washing at infrequent intervals only and whose faceyarn is made of at least 20% of viscose artificial textile fiber of average diameter between 37 and 75 microns, less than 20% of whose crosssectional periphery is on a radius of curvature between 4 microns and 0.4 micron, whose crosssectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on a radius less than 4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain dust.

12. A house-furnishing pile fabric whose pile yarn is made of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 27 and 75microns, less than 20% of whose cross-sectional periphery is on a radius of curvature between 4 microns and 0.4 micron, whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on a radius less than 4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain dust.

13. A house-furnishing pile fabric whose pile yarn is made of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 27 and 75 microns and which fiber substantiallyv conforms to the condition:

11,310,000 DenierXspecific gravity 14. A pile fioor covering fabric such as a carpet or rug whose-pile yarn is made of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 27 and 75 microns, less than 20% of whose cross-sectional periphery is on a radius of curvature between 4 microns and 0.4 micron, whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are ,on a radius less than 4microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain dust.

15. A floor covering fabric whose face yarn is made of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 27 and 75 microns and which fiber substantially conforms to the condition:

Surface per gram in 11,310,000

Square centimeters v DenierXspecific gravity 16. A pile floor covering fabric such as a carpet or rug whose pile yarn is made of at least 20% of viscose artificial textile fiber of average diameter between 37 and 75 microns, less than 10% of whose cross-sectional periphery is on a radius of curvature between 6 microns and 0.4 micron, whose cross-sectional ,periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any arc on a radius less than 4 microns, and substantially free'from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain-dust.

17. A pile floor covering fabric such as a carpet or rug whose pile yarn consists of ar- Surface per gram in square centimeters tificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 37' and 52 microns, less than ofwhose' cross-sectional periphery is on a radius of curvature between 6 -microns and 0.4 micron, whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on a radius less than 4 microns, and substantially free from reentrant curves'of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain dust.

18. An upholstery fabric whose face yarn is made of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and.

naturally-occurring-protein fibers and of average diameter between 27 and microns, less than 20% of whose cross-sectional periphery is on a radius of curvature between 4 microns and 0.4 micron, whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any arc .ona radius less than 4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain dust.

19. An upholstery fabric whose face yarn is made of at least 20%- of viscose artificial textile fiber of average diameter between 27 and 75 microns, less than 20% of whose cross-sectional periphery is on a radius of curvature between 4 microns and 0.4 micron, whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on a radius less than 4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn g a reduced tendency to retain dust.

20. An upholstery fabric whose face yarn is made-of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 27 and 75 microns and which fiber substantially conforms to the condition:

whose cross-sectional periphery is on a radius of curvature between 4 microns and 0.4 micron, whose, cross-sectional periphery is substantially free from reentrant curves which have a depth face yarn possessing a reduced tendency to re- 22. A suiting whose face yarn is made of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-proteinfibers and of average diameter between 27 and 75 microns and which fiber substantially conforms to the condition:

Surface per gram in 11,310,000 square centimeters n i rx ifi gravity 23. A fabric which is of the type subject to washing at infrequent intervals only, and whose face yarn is made of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 27 and 75 microns,

whose cross-sectional periphery is asubstantially smooth closed curve substantially free from curves on a radius less than 4 microns, the face yarn possessing a reduced tendency to retain dust.

24. A fabric which is of the type subject to washing at infrequent intervals only, and whose face yarn is made of at least 20% of regenerated cellulose artificial textile fiber of average diameter between 27 and 75 microns, whose crosssectional periphery is a substantially smooth closed curve substantially free from curves on a radius less than 4 microns, the face yarn ing a reduced tendency to retain dust.

25. A fabric which is of the type subject to washing at infrequent intervals only and whose face yarn is made of at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 27 and 75"microns,

less than 20% of whose cross-sectional periphery is on a radius of curvature between 4 microns and 0.4 micron, whose cross-sectional periphery is undulating but substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on a radius less than 4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn duced tendency to retaindust.

26. A fabric which is of the type subject to washing at infrequent intervals only and whose face yarn is made of at least 20% of artificial staple fiber of the class of cellulose fibers and naturally-occun'lng-protein fibers and of average diameter between 27 and 75 microns, less than 20% of whose cross-sectional periphery is ona radius of curvaturebetween 4 microns and0.4 micron, whose. cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on a radius less than 4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn g a reduced tendency to retain dust.

27. A fabric which is of the type subject to washing at infrequent intervals only and whose face yarn is made of at least 20% of viscose artificial staple fiber of average diameter between 27 and 75 microns, less than 20% of whose depth greater than 0.5" micron whose mouth' opening is less than five times the depth, the

face yarn g aredheed tendency to re- 28. A fabric which is of the type subject to protein filaments and of average diameter between 27 and '75 microns, less than 20% of whose cross-sectional periphery is on a radius of curvature between 4 microns and 0.4 micron, whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on a radius less than 4 microns, and substantially free from reentrant curves ofdepth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain dust.

29. A fabric which is of the type subject to washing at infrequent intervals only and whose face yarn is made of a blend of a coarse animal fiber and at least 20% of artificial textile fiber of the class consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 27 and microns, less than 20% of whose cross-sectional periphery is on a radius of curvature between 4 microns and 0.5 micron, whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any are on a radius less than 4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, the face yarn possessing a reduced tendency to retain dust.

30. The process of substantially reducing the dust retention by a fabric which is of the type subjectv to washing at infrequent intervals o y. which comprises forming yarn containing at least 20% of artificial textile fiber of theclass consisting of cellulose fibers and naturally-occurring-protein fibers and of average diameter between 2'? and '15 microns, less than 20% of whose cross-sectional periphery is on a radius of curvature between 4 microns and 0.4 micron,-

whose cross-sectional periphery is substantially free from reentrant curves which have a depth greater than 0.5 micron and which curves include any arc on a radius less than 4 microns, and substantially free from reentrant curves of depth greater than 0.5 micron whose mouth opening is less than five times the depth, and incorporating the yarn as the face yarn of a fabric subject to washing at infrequent intervals only.

31. The process of substantially reducing the dust retention by a fabric which is of the type subject to washing at infrequent intervals only, which comprises forming a yarn consisting chiefly of artificial staple fiber of the class consi ting of cellulose fibers and naturally-occurring-protein fibers and ofaverage diameter between 3'! and 75 microns, whose cross-sectional periphery is a substantially smooth closed curve substantially free from curves of radius than 4 microns, and incorporating the yarn as'the face yarn of a fabric subject to washing at infrequent intervals only.

CHARLES nmmv msmm), 2n. 

